1. Field of the Invention
This invention relates to a method of forming a bottom stop on a slide fastener having coupling elements fabricated from continuous filaments, such as those of the helicoidal-coil or zigzag type. More specifically, the invention concerns a method of forming a bottom stop by ultrasonically welding together several interengaged lowermost scoops of the continuous coupling elements.
2. Prior Art
Bottom stops have been suggested which are formed by fusing or welding together several interengaged lowermost scoops of continuous coupling elements on stringer or carrier tapes, as by vibrating the interengaged scoops at ultrasonic frequencies, as disclosed for example in U.S. Pat. No. 3,263,291. Such bottom stops, as herefore made, are unsatisfactory for the following reason.
Coupling elements in general are capable of withstanding crosswise tensile stresses by virtue of their interengaged heads. The interengaged coupling elements of the helicoidal-coil and zigzag configurations, however, are susceptible to ready displacement when subjected to crosswise compressive stresses. This is because gaps exist between the coupling heads of each continuous coupling element and a filler (i.e., a cord or like core in the case of coil type coupling elements and a longitudinal stringer tape edge in the case of zigzag type coupling elements) received in the other continuous coupling element.
From the standpoint of this invention, the scoops of the continuous coupling elements can be considered to be somewhat loosely interengaged by reason of the existence of the above noted gaps. Thus, when vibrated ultrasonically, the several interengaged lowermost scoops of the coupling elements do not develop sufficient frictional heat to coalesce into a firmly united bottom stop. The efficiency of the ultrasonic welding operation is also poor.